The invention relates generally to municipal or industrial liquid clarification tanks, and more particularly to a slurry level control system for xe2x80x9cClariConexe2x80x9d type upflow liquid clarification tanks that have a lower section, a conically-shaped flocculation/clarification section, a liquid outlet, a slurry outlet, and a valve on the slurry outlet.
Conventional Clarifiers
In conventional water treatment clarifiers, the main purpose of the clarifier is to separate the solids from the water. The solids settle to the bottom of the clarifier and accumulate to form a slurry layer in the bottom of the clarifier. The slurry level in the clarifier does not directly affect the water treatment process.
Excess slurry is periodically wasted from the clarifier, and the slurry level is typically maintained above some minimum height to prevent clear water from being discharged during those times. The slurry level is also typically maintained below a certain maximum height to prevent the slurry from becoming septic and to prevent the slurry from building up to such a high level that slurry is carried over the weirs with the effluent. The desired slurry level is not dependent on such treatment parameters as the flow rate or the raw water turbidity.
Conventional Solids-Contact Clarifiers
In conventional solids-contact clarifiers, or reactor clarifiers, it is important for the solid particles in the raw water to be mixed with the previously-formed slurry to allow the small raw water particles to come into contact with, and stick to, the previously-formed slurry particles. The larger, denser slurry particles settle more quickly and allow the clarifier to be operated at a higher hydraulic loading rate. Coagulating and flocculating chemicals are typically added to the raw water to enhance this particle agglomeration process. In this type of clarifier, the slurry layer is an important part of the treatment process.
These clarifiers typically include some type of mechanical slurry recirculation equipment to mix the previously-formed slurry particles with the fine particles in the raw water. This typically occurs in a central flocculation chamber within the clarifier. The central flocculation chamber is typically sized to provide a certain minimum detention time for the water being treated. The slurry level is typically kept at or above a certain minimum level to make sure that the slurry recirculation equipment can pick up the settled slurry and mix it with the incoming raw water. Similarly, a minimum slurry level may be kept to ensure that the water exiting the central flocculation area has to pass through the bed of previously formed slurry to allow further contact between the fine particles in the raw water with the larger, denser slurry particles.
In both cases, the slurry level is maintained at a certain elevation that is determined by the recirculation equipment dimensions or by the central flocculation area discharge elevation. The desired slurry level is not dependent on such treatment parameters as the flow rate or the raw water turbidity.
xe2x80x9cClariConexe2x80x9d(trademark) type clarifiers
In a ClariCone type clarifier, the water being treated flows upwards through a cone-shaped vessel. The vertical velocity of the water (the rise rate) changes as the water moves upwards through the conical portion of the clarifier. The rise rate is high in the small diameter, lower end of the conical portion of the clarifier. Particles in the water settle through the water at a settling velocity (settling rate) that depends on the size and density of the particles. Large, dense particles settle at a higher rate, and can accumulate at higher rise rates (at lower elevations within the conical portion of the clarifier) than smaller, less dense particles.
A slurry layer accumulates in the lower portion of the conical section of the clarifier. The volume of the slurry layer determines the amount of time that it takes for the water to flow through the slurry layer. This detention time within the slurry layer is important in that it provides the time for fine particles in the raw water to contact, and adhere to, the larger, denser, previously formed slurry particles in the slurry layer. A thicker slurry layer minimizes the amount of treated water that is wasted along with the slurry when excess slurry is wasted from the clarifier.
The top of the slurry layer is maintained at an elevation that will provide at least a certain desired minimum slurry layer detention time and at an elevation that will prevent the rise rate of the water at the slurry layer surface from exceeding a certain desired maximum value. The elevation at which these values occur changes with the flow rate of the water being treated. The desired slurry layer detention time and the desired maximum rise rate also change with such treatment parameters as the water temperature and the raw water contaminant level.
Conventionally, the slurry level in a ClariCone-type clarifier is adjusted manually by the operator, based on observations of the slurry layer.
Like previously-known ClariCone-type clarifiers, a tank within the scope of the present invention has a lower section, a conically shaped flocculation/clarification section, a liquid outlet, a slurry outlet, and a valve on the slurry outlet. Unlike previously-known ClariCone-type clarifiers, it also has a level sensor, a flow-condition detector, and a controller that work together for automatic control of the valve.
The lower section of the tank includes a liquid inlet through which liquid to be clarified enters the tank. The flocculation/clarification section of the tank is above the lower section, and has an outwardly-sloping sidewall. A liquid outlet near the top of the flocculation/clarification section is used to draw off clarified liquid.
The slurry outlet is within the flocculation/clarification section. It is connected to the valve that can be used to drain slurry from the outlet. The slurry outlet may comprise a slurry concentrator, and may be fixed in position.
The flow-condition detector is used to provide information that is used in determining when the slurry has accumulated to a level where it is desirable to waste, or blow down, excess slurry from the tank. This level is called the xe2x80x9cblow-down level.xe2x80x9d The flow condition detects a condition of the flow through the clarification tank. For example, the flow-condition detector may be a flow rate sensor. Alternatively, the flow-condition detector may measure the quality of the water, such as its temperature, turbidity, or hardness. The detector sends the input concerning the detected condition to the controller.
The controller uses the input from the flow-condition detector to determine the desired blow-down level. The desired blow-down level may, for example, be based on an incremental height above a calculated minimum slurry level, such as 10% or 18xe2x80x3 above a calculated minimum slurry level. Other factors may also be considered in determining the desired blow-down level. For example, the desired blow-down level may vary depending upon the configuration and dimensions of the tank, and the desired detention time and rise rate.
The level sensor detects the height of slurry in the tank, and the controller opens the valve when the slurry level reaches the desired blow-down level determined by the controller.
The closing of the valve can be controlled in many ways. The valve can be set to close after a predetermined fixed period of time. Another option is to provide an adjustable drain timer. Yet another option is to set the valve so that is closes automatically when the level sensor detects that the slurry level has fallen to a predetermined or calculated blow-down termination level.